By radioligand assay, receptors for protein and peptide hormones are generally ascribed to cell membrane fractions. By immunocytochemistry, most of these receptors are found intracellularly. The present project explores a physiologic significance of these intracellular receptors using as a model, recpetors for luteinizing hormone-releasing hormone (LHRH) on secretory granules of pituitary gonadotrophs. To investigate the relationship of these receptors with other functions of the cell, LHRH, cAMP, cGMP and estradiolbenzoate (E2), as well as receptors reactive with these four ligands in vitro (free receptors), are assayed in rats at varying time intervals after intravenous injection of LHRH and E2 and during various phases of the estrus cycle. The subcellular distribution of injected hormone and cell-generated cyclic nucleotides is correlated with morphologic observation of degranulation and castration cell formation. The time sequence of appearance and disappearance of cell membrane and intracellular hormone binding will be interpreted in relation to release of granule contents, recycling and regeneration of hormone receptors, and generation and distribution of cyclic nucleotides. In addition, light microscopic studies will correlate the cellular distribution of LHRH and steroid receptors in brain with each other and in the pituitary with FSH and LH contents.